The most common form of data storage for computers, hard disk drives in general share a similar basic head-disk assembly (HDA) structure. The actual disks, or platters, are traditionally made of a light alloy, glass or ceramic material coated with a very thin layer of magnetic medium. The magnetic layer has domains of magnetized areas oriented to store information through the use of read and write heads. Multiple platters on a disk drive are separated by disk spacers and are affixed to a rotatable spindle attached to a motor. The spindle is able to rotate all of the platters in unison. The motor is connected to a feedback loop to ensure the correct rotating speed, which may be on the order of 3,600 to 15,000 rpm.
The read and write heads are typically attached to a single actuator arm that moves the heads around the platters as needed. When the platter is spinning, the heads ride on a cushion of air. When the platter is at rest, the heads come to rest, or park, at a predetermined landing zone or parking area. This parking area may include ramps used to move the heads off of the surface of the disk. In order to avoid damage to the disks, heads, and stored data, precise control of the head actuator is critical in any head-disk assembly. Stepper motor actuators are known in the arts. Stepper motor actuators move the actuator over the platters in predefined steps. Stepper motor actuators are generally slow, blind to track position, incapable of meeting current track pitch requirements, prone to misalignment, and are sensitive to variations in temperature.
In an attempt to address these problems, voice coil actuators, or servos, have been developed to control head movement. The voice coil is moved relative to a permanent magnet based on the magnitude of current flowing though it. Voice coil actuators get feedback as to position over the platter, assuring that the proper tacks are read, are not constricted to discrete steps, and are less sensitive to temperature changes. Voice coil actuators generally have small cables or springs designed to drag, with some reliance on windage, the heads into a park position when the drive is powered down. This approach is not desirable however, because it requires additional current to keep the heads on track over the middle of the disk. Additional problems arise in the control of the actuators. Frequent recalibration is necessary for servo motors to maintain precision. While tracking, position feedback from the disk surface is required. Distances between the parts of a disk drive, for example the heads and the platters, are extremely small. Therefore abrupt or imprecise movements of the actuator when parking or unparking can result in damage to the heads, data, or disks.
Problems with imprecisely controlled movements in particular can arise due to the variability in the relative velocity and position of the actuator and platter. Prior art actuator control has entailed providing a fixed current to the actuator motor. This approach has the disadvantage of causing the actuator motor to continuously accelerate for as long as the actuator motor current remains uninterrupted. Excessive acceleration of the actuator motor can cause damagingly abrupt actuator movements as in slamming into a fixed “crash stop” at an excessive velocity. An alternative prior art approach has been to provide a constant voltage across the actuator motor. This approach has the disadvantage of being unresponsive to changes in the load on the actuator motor. Load changes may occur during normal operation for a number of reasons, for example, when the heads are moved up a parking ramp, the load on the motor tends to increase. The increased load may cause the head to move more slowly, causing an overall loss of speed in the operation of the actuator arm. Efforts to avoid abrupt movements have traditionally relied on sampling the motor voltage using an analog circuit, and attempting to adjust the voltage applied to the actuator motor accordingly. The lack of flexibility and area requirements of the analog circuitry hamper the effectiveness of such efforts.
Due to these and other problems with controlling the movement of actuators in HDAs, it would be useful and desirable in the arts to increase the speed and accuracy of monitoring for improved control. It would be particularly advantageous if improvements to actuator voltage control retract functions also contributed gains in terms of minimization of chip area and reductions in cost.